nerve gas

nerve gas The term ‘nerve gas’ implies substances manufactured with the intent of killing populations or decimating armies in time of war. They have never been used as such in a major world war. The idea that an enemy could stealthily pollute the air with a gas that when breathed would cause death, without affecting property, is horrific and the use of poisonous gases in this way is rightly banned by international treaty. Nevertheless the whole population of Britain, including children and babies, were issued with respirators (‘gas masks’) in World War II. Chlorine gas shells were used by the German Army in World War I, but it proved a hazardous business as wind changes often blew the gas back across the lines from where it came. Chlorine is not a nerve gas, and many soldiers survived the attacks, but were left with lifelong bronchitic type conditions because of the damage to the lungs.

The nerve gases are not really gases, and were designed to be sprayed as aerosols over enemy territory, the fine droplets being absorbed through the skin following contact, either directly from the air or by touching surfaces on which the droplets rested. Thus ‘gas masks’ were of no use, as the fine droplets of poison could easily be distributed without detection and persisted for some time after the air raid. The principle upon which the nerve gases worked was to inhibit a particular enzyme in the body, namely acetylcholinesterase, which rapidly breaks down the neurotransmitter, acetylcholine. Acetylcholinesterase is associated with many nerve endings, both in the brain and in the periphery. Acetylcholine is the neuromuscular transmitter at motor nerve endings in skeletal muscle, in ganglia (the relay stations of the autonomic nervous system), and at all parasympathetic nerve endings on the tissues they affect. The parasympathetic nervous system controls many vegetative functions of involuntary organs and is active under normal sedentary conditions, promoting secretions for example of saliva, and the digestive enzymes in the gut, and also, through the vagus nerve, serving to slow the heart rate and lower the blood pressure. If the enzyme which destroys the transmitter is itself inhibited then the transmitter will persist for much longer, so that overactivity at all situations where acetycholine is involved will be apparent. The consequences therefore are increased salivation, tears, and gastrointestinal and bronchial secretions; bronchoconstriction; slowing of the heart and a fall in blood pressure; constriction of the pupil; and a fall in pressure in the eyeball. Effects caused within the brain lead eventually to convulsions, loss of consciousness, and respiratory failure. It is of course possible to treat some of these symptoms and it is particularly important to give an atropine-like drug (which counteracts the effects of acetylcholine) to prevent the heart stopping altogether.

The nerve gases, and there are many of them, are pentavalent phosphorus compounds (known as organophosphorus compounds), such as dyflos (di-isopropyl fluorophosphonate) and parathion, which react irreversibly with acetylcholinesterase, so the effects are long-lasting, requiring the body to synthesize new enzyme, which takes several weeks. The reason is that the organophosphorus compounds transfer a phoshate group to the enzyme, making it inactive. A little later there is a ageing process when the phosphorylated enzyme changes its structure. It is only before the ageing process occurs that it is possible to remove the phosphate group with nucleophilic drugs, such as pralidoxime. Clearly, treatment of any kind would be unlikely during a nerve gas attack with thousands of people affected.

Nerve gases have proved to have important peacetime uses. They are widely used as insecticides. The compounds can penetrate the insect cuticle as they can human skin. Occasionally farmers contaminate themselves when diluting the concentrate for spraying and need to be treated as described above. Dyflos is used in eyedrops to treat glaucoma (raised intraocular pressure); applied locally in the eyes it produces no effects elsewhere in the body.

Alan W. Cuthbert

See also chemical warfare; poisons.